A preliminary, closed-loop, television tracking system has been developed to measure the motion of beating mouse myocardial cells in intro. The performance of these cells is used to monitor the presence of circulating myocardial depressant substances present in the blood serum of patients with septic shock. The beating cells are placed in a petri dish which is mounted in an inverted microscope. The cells are flushed with a solution of 4 micron latex spheres which adhere to the cell walls. Due to the differences in the indices of refraction between the solution and the spheres, the spheres appear dark in the microscope field. A miniature CCD (charge coupled device) television camera is used to display the image from the microscope on a television monitor. An electronic targeting circuit superimposes a "target window" on the monitor. The petri dish is scanned by the operator to select a target bead moving with a cell. The camera is then rotated until the bead is moving horizontally along the television raster lines. The X-Y manipulators on the microscope are used to locate the moving bead in the displayed target window. At this point, the track switch is closed to complete the feedback loop and the target window "locks" onto the bead. The error voltages generated to track the bead are a measure of the displacement of the beating cell. After a control run, serum from a patient is added to the medium surrounding the cells and the activity monitored. If a myocardial depressant factor is present, the beating of the cells is affected. A portion of each run is sampled and stored by a microcomputer.